1. Field of the Invention
The present invention relates to a hologram-recording method, an optical recording medium, and a hologram-recording device, and in particular to a hologram-recording method for recording a volume hologram in an optical recording medium, and an optical recording medium and a hologram-recording device favorably used in the recording method.
2. Description of the Related Art
Forgery and alteration of ID and credit cards and the like have been emerging as a serious social problem recently. One of the reasons for that is the progress in copying technology, which enables duplication of the hologram seals adhered for prevention of forgery.
Conventional hologram seals, which are duplicated from a master hologram, generally employ a rainbow hologram generated by surface relief pattern. For example, a press die is prepared from a master relief hologram having a surface relief pattern; a great number of hologram seals having the same pattern are produced by heat-pressing process using the die; and such hologram seals are adhered to display media such as cards.
However, duplication of the surface-relief-based pattern can be performed relatively easily by using the current copying technology. As a result, the forgery preventing effect of hologram seals is diminishing day by day.
Recently, a method of on-demand hologram printing has been proposed [Japanese Patent Application Laid-Open (JP-A) No. 2000-263910] for making the forgery more difficult. The method provides display media such as cards with, for example, a hologram adapted to individual information such as ID number, production lot, and the like, thereby making the hologram more effective for preventing alteration and thus more reliably ensuring the security of the cards. The on-demand holograms are usually printed by using a hologram transfer foil ribbon (JP-A No. 2000-211257).
However, the on-demand holograms, which employ a hologram transfer foil ribbon, have the following problems: it is possible for an outsider to duplicate a similar hologram by using a hologram not printed on a display medium but remained on the transfer foil ribbon (thus it is necessary to put the transfer foil ribbon after use under strict control for prevention of forgery); and as the transfer foil ribbon is a relief hologram, it is possible to forge the hologram by examining the surface relief structure thereof.
On the other hand, the holograms most effective in preventing forgery and alteration are volume holograms represented by Lippmann hologram and the like. These volume holograms, which form a refractive index modulation grating in an optical material such as photopolymer instead of forming a surface relief pattern, are much more resistant to forgery.
As volume holograms, there are two types, i.e. a reflection type hologram, in which signal beam and reference beam are irradiated and recorded in opposing directions relative to a hologram recording medium, and a transmission type hologram in which signal beam and reference beam are irradiated and recorded in the same direction relative to a hologram recording medium. In both of the two, the signal beam and the reference beam interfere with each other in the hologram recording medium, and the resulting interference fringes induce refractive index changes or absorptivity changes in the hologram recording medium, thereby recording the hologram.
However, in a recording method of a conventional volume hologram, the signal beam formed as a pattern, according to the images or characters to be recorded, is made to be incident onto the hologram recording medium, and the reference beam is irradiated so as to cover the irradiation area of the signal beam. According to this method, reference beam is irradiated alone onto areas where the pattern of the signal beam is absent, thereby forming an undesired diffraction grating from multiple interference or the like due to the layer structure constituting the hologram recording medium. This undesired diffraction grating creates undesired diffracted light which deteriorates the finish of the reproduced image, causing problems.